In the coated glass industry it is desirable to apply one or more thin layers of coating materials to one or both surfaces of glass sheets to provide desired characteristics to the finished coated glass product. For example, it is often desirable to apply infrared-reflective coatings and/or other multilayer coating systems to provide desirable characteristics related to transmittance, emissivity, reflectance, durability, color, photocatalysis and/or chemical resistance.
It can be argued that sputter deposition is frequently the most effective method of forming thin films on substrates. Compared with other thin-film formation methods such as vacuum evaporation, electroplating, electroless plating, and chemical vapor deposition, sputter deposition can allow for more variety with respect to selection of film materials, higher purity and controlled composition of the film, greater adhesive strength and homogeneity of the film, and greater control of the film thickness.
In conventional sputtering deposition systems, a substrate is generally conveyed through a vacuum chamber. The chamber contains a target made of or including sputterable material on its outer surface. Sputtering occurs when energized ion particles impinge on the target's surface, causing liberation of target atoms. In reactive sputtering, such liberated target atoms combine with reactive gas atoms to form a dielectric material, which is in turn deposited onto the substrate.
The deposition process often involves direct current (“DC”) reactive sputtering. To achieve such sputtering, an electrical field is created in the vacuum chamber. Typically, a DC negative voltage is applied to the target in creating a cathode and a DC positive voltage is applied to other apparatus provided in the chamber at a location spaced away from the target in creating an anode. As such, the electrical field is created between the cathode and the anode. One or more gases are introduced into the vacuum chamber. Electrons in the electrical field are accelerated and gain enough energy to ionize the gas atoms and create glow discharge plasma. For example, an inert gas, such as argon, can be used in producing such plasma. Additional gases may also be introduced to chamber if desired, such as one or more reactive gases. As the ions are drawn to the target by the electric field of the cathode sheath, they bombard the target and liberate atoms. As described above, when such bombarding and liberating is conducted in the presence of a reactive gas such as oxygen or nitrogen, a reactive product of coating material (i.e., the product of the liberated target atoms and the reactive gas atoms) is generated, much of which is deposited on the substrate. For example, introducing a reactive gas such as oxygen or nitrogen to the chamber forms an oxide or nitride with the liberated target atoms.
Most of the voltage drop between the cathode and anode occurs over a distance of about 2 mm proximate to the cathode. This is due to the tendency of the plasma to form an electrical barrier commonly called the cathode sheath. A smaller sheath, the anode sheath, also forms proximate to the anode. As such, the bulk of the plasma has only a very weak electrical field that results from a gradient in charge density. The gradient is due to the relatively high density where the electrons are generated (at or near the cathode) and the relatively low density where the electrons are lost (at the anode). In some systems, the chamber walls or another conductive member positioned away from the target may be chosen to function as the anode.
As is known, with the ions being withdrawn from the plasma, an excess of electrons are left. It is a basic requirement that the plasma remains substantially charge neutral. It is also a requirement for the power source that there is equal electrical current going through the positive and negative output poles. Hence, the anode is provided to absorb the excess electrons and return them to the power source. However, this process can be compromised over time, as described below.
One disadvantage of DC reactive sputtering is a disappearing anode issue. This issue occurs when the anode gradually becomes coated with dielectric material. As is known, dielectric material often coats every surface in the chamber (including the substrate) and eventually coats the anode. When an area on an anode becomes coated, the conducting path for electrons in that area is compromised, causing the coated area to lose its ability to conduct electrons, and thereby cutting off the current path for electrons emitted from the process. When this happens, the electrons typically conduct to an uncoated area on the anode or, if the anode is covered by sputtered dielectric material, as quite often is the case, a less coated area on the anode.
Such less coated area is often a “hot spot” of the anode, i.e., an anode area that has a thinner coating of dielectric material than the other areas on the anode, so as to provide a path of least resistance for conducting electrons. Electrons flood to this hot spot. As electron current converges on this hot spot, this area heats up and vaporizes the coating in this area. This improves the conduction path to this spot even further. However, when such a dominant conduction path is established, the electron conduction in this spot becomes intense relative to the other coated areas on the anode and the effective geometry of the anode changes. In turn, as the anode geometry changes, the electric field (i.e., charge gradient) in the chamber also shifts. This shift has been found to cause changes in the film deposition pattern of the system. As a result, there are undesirable changes in film uniformity on the substrates.
Methods have been used to address the disappearing anode issue. Some methods involve using dual sputtering targets with AC power or bi-polar pulsed DC power instead of normal DC power. In these cases, one target acts as the sputtering cathode, which is sputtered clean during every other half-cycle and then acts as the anode during the other half-cycle. While these methods are effective, they require replacing existing power supplies and adding additional target assemblies. As a result, the methods requires additional capital investment, inventory and maintenance. Also, an AC pair of targets only has about the same power capability as a single DC sputtering target. As such, when compared to such DC systems, an AC system generally only provides half the sputtered deposition capability on a per-target basis.
Another method involves using a gas manifold as an anode. Generally, the gas manifold is isolated from ground and connected to a positive side of the output of a power supply. Gas enters into the manifold through an inlet and exits through one or more outlets. The pressure around the outlets is generally higher as compared to the rest of the sputtering chamber. When sputtering occurs, a majority, if not all, the outer manifold surfaces are eventually coated with a dielectric material. However, the gas streaming outward from the manifold outlets prevents the sputtered material from coating over the outlets. The outlets then become the preferred conduction paths for electrons. These paths are somewhat enhanced due to the in-streaming electrons colliding with the out-streaming gas, which is at a locally elevated pressure. These collisions produce plasma plumes at each outlet. Since the plasma is electrically conductive, each plume serves as a virtual anode that cannot be coated.
One issue with the above gas manifold method is that the outer surfaces of the manifold (i.e., the anode) become coated with dielectric material from the sputtering process, as described above. This coating of the gas manifold can affect the overall functioning of the manifold as the anode. For example, at an initial stage of the deposition process, when the gas manifold is generally clean from any sputtered dielectric material, any surface of the gas manifold will be anodic, serving as a conduction path for electrons in the chamber back to the power source. However, as the outer surfaces of the gas manifold become coated with the sputtered dielectric material over time, the electrons gradually shift to conduction paths involving uncovered portions of the anode. Thus, the initial attractive effect established between the anode and the electrons is altered. This, in turn, changes the manner in which the plasma within the chamber is maintained, and as a result, can cause an undesirable shift in the coating function of the chamber.
As is known, a typical manifold in a sputtering system includes a tube with an inlet at one point and holes (e.g., drilled periodically along the tube's length) that act as the outlets. However, because there is a different gas conductance between the manifold inlet and each outlet, the distribution of gas is non-uniform. This can be modified, to a degree, by changing the relative size of the tube diameter and the outlet diameter. As the tube diameter is increased, the conduction path to each outlet is improved, leading to an improved gas distribution. Alternatively, reducing the conductance through the outlets helps improve distribution by increasing the pressure inside the tube, which enhances distribution of the gas. However, either of these modifications has the adverse effect of increasing the gas capacity of the tube. The capacity, i.e., the total amount of gas in the manifold, can be thought of as the product of volume and pressure. By increasing the gas capacity of the tube, response time is slowed for any closed loop feedback control used to control the gas. This is because all the gas in the manifold must exit the manifold before any change can take effect in the process chamber.
What are needed are apparatus and/or methods for overcoming the limitations described above.